Agent Skills: ln-773-cors-configurator

ln-773-cors-configurator

Type: L3 Worker Category: 7XX Project Bootstrap Parent: ln-770-crosscutting-setup

Configures Cross-Origin Resource Sharing (CORS) policy with security-first approach.

Overview

| Aspect | Details | |--------|---------| | Input | Context Store from ln-770 | | Output | CORS configuration with environment-specific policies | | Stacks | .NET (ASP.NET Core CORS), Python (FastAPI CORSMiddleware) |

Phase 1: Receive Context

Accept Context Store from coordinator.

Required Context:

• STACK: .NET or Python
• PROJECT_ROOT: Project directory path
• ENVIRONMENT: Development or Production

Idempotency Check:

• .NET: Grep for AddCors or UseCors
• Python: Grep for CORSMiddleware
• If found: Return { "status": "skipped" }

Phase 2: Analyze Project Structure

Determine frontend configuration.

Detection Steps:

1. Check for frontend in same repository (/frontend, /client, /web)
2. Read .env or appsettings.json for CORS_ORIGINS
3. Identify common frontend ports (3000, 5173, 4200)

Detected Frontend Origins:

| Framework | Default Port | Origin | |-----------|--------------|--------| | React (CRA) | 3000 | http://localhost:3000 | | Vite | 5173 | http://localhost:5173 | | Angular | 4200 | http://localhost:4200 | | Next.js | 3000 | http://localhost:3000 |

Phase 3: Decision Points

Q1: Allowed Origins

| Environment | Strategy | |-------------|----------| | Development | Allow localhost origins (configurable) | | Production | Explicit origins from environment variables only |

Security Warning: Never use * (wildcard) with credentials.

Q2: Allowed Methods

| Method | Default | Notes | |--------|---------|-------| | GET | ✓ Yes | Read operations | | POST | ✓ Yes | Create operations | | PUT | ✓ Yes | Update operations | | DELETE | ✓ Yes | Delete operations | | PATCH | Optional | Partial updates | | OPTIONS | ✓ Yes | Preflight requests (automatic) |

Q3: Credentials Support

| Scenario | AllowCredentials | Notes | |----------|------------------|-------| | Cookie-based auth | ✓ Yes | Required for cookies | | JWT in header | ✗ No | Not needed | | OAuth2 | Depends | Check documentation |

Warning: AllowCredentials = true prohibits * origin.

Q4: Preflight Cache Duration

| Environment | MaxAge | Rationale | |-------------|--------|-----------| | Development | 0 | Immediate config changes | | Production | 86400 (24h) | Reduce preflight requests |

Phase 4: Generate Configuration

.NET Output Files

| File | Purpose | |------|---------| | Extensions/CorsExtensions.cs | CORS service registration | | appsettings.json (update) | Origins configuration | | appsettings.Development.json (update) | Dev origins |

Generation Process:

1. Use MCP ref for current ASP.NET Core CORS API
2. Generate CorsExtensions with:
   • Development policy (permissive)
   • Production policy (restrictive)
   • Environment-based policy selection
3. Update appsettings with CORS:Origins

Registration Code:

builder.Services.AddCorsPolicy(builder.Configuration);
// ...
app.UseCors(builder.Environment.IsDevelopment() ? "Development" : "Production");

Python Output Files

| File | Purpose | |------|---------| | middleware/cors_config.py | CORS middleware configuration | | .env (update) | CORS_ORIGINS variable |

Generation Process:

1. Use MCP ref for FastAPI CORSMiddleware
2. Generate cors_config.py with:
   • Origin parsing from environment
   • Method and header configuration
   • Credentials handling
3. Update .env with CORS_ORIGINS

Registration Code:

from middleware.cors_config import configure_cors
configure_cors(app)

Phase 5: Validate

Validation Steps:

1. Syntax check:

   • .NET: dotnet build --no-restore
   • Python: python -m py_compile middleware/cors_config.py

2. CORS test:

   # Test preflight request
   curl -X OPTIONS http://localhost:5000/api/test \
     -H "Origin: http://localhost:3000" \
     -H "Access-Control-Request-Method: POST" \
     -v
   

3. Verify headers:

   • Access-Control-Allow-Origin: Should match request origin
   • Access-Control-Allow-Methods: Should list allowed methods
   • Access-Control-Allow-Credentials: true (if enabled)
   • Access-Control-Max-Age: Cache duration

Security Checklist

Before completing, verify:

• [ ] No wildcard * origin in production
• [ ] Explicit allowed methods (not AllowAnyMethod in prod)
• [ ] Credentials only if needed
• [ ] Origins from environment variables in production
• [ ] Preflight caching enabled in production

Return to Coordinator

{
  "status": "success",
  "files_created": [
    "Extensions/CorsExtensions.cs"
  ],
  "packages_added": [],
  "registration_code": "builder.Services.AddCorsPolicy(configuration);",
  "message": "Configured CORS with Development and Production policies"
}

Reference Links

• ASP.NET Core CORS
• FastAPI CORS
• MDN CORS

Critical Rules

• Never use wildcard * origin with credentials — security violation per CORS spec
• Production origins from environment variables only — no hardcoded URLs in code
• Separate Development and Production policies — permissive locally, restrictive in production
• Idempotent — if AddCors/UseCors or CORSMiddleware exists, return status: "skipped"
• Enable preflight caching in Production — MaxAge 86400 (24h) to reduce OPTIONS requests

Definition of Done

• Context Store received (stack, project root, environment)
• Frontend origins detected (port/framework auto-detection)
• User decisions collected (origins, methods, credentials, cache duration)
• CORS configuration generated with environment-specific policies
• Security checklist verified (no wildcard + credentials, explicit methods, env-based origins)
• Syntax validated (dotnet build or py_compile)
• Structured JSON response returned to ln-770 coordinator

Version: 2.0.0 Last Updated: 2026-01-10